After a short three-month beta program, Microsoft is officially releasing Microsoft Security Essentials (MSE), its free, real-time consumer antimalware solution for fighting viruses, spyware, rootkits, and Trojans. MSE is yet another layer of defense the company is offering to help its customers fight the threats that plague Windows PCs.

Microsoft Security Essentials is available for Windows XP 32-bit (8.61MB), Windows Vista/7 32-bit (4.28MB), and Windows Vista/7 64-bit (4.71MB). The final build number is 1.0.1611.0. Microsoft warns that MSE should not be installed alongside any other antimalware application. Indeed, MSE's installer disables Windows Defender completely, which makes sense as it is a sort of superset to Windows Defender. It builds upon Windows Defender by offering both real-time protection and on-demand scanning for all types of malware.

Although you won't be asked for personal information or to register for anything, you'll need to pass the Windows Genuine Advantage validation to install MSE. In short, while Microsoft's security updates are available for free to pirates, active protection from Redmond isn't.

One might notice the "Essentials" branding that Microsoft is clearly pulling from Windows Live Essentials. MSE won't be included in WLE, however, even though non-Windows Live applications like Silverlight are included in the suite. Furthermore, there is no download link for MSE included in the final release of Windows 7, but there is for Windows Live Essentials. This is a curious decision given that Redmond wants to push MSE out to all those who currently do not have a security solution (between 50 and 60 percent of Windows users, according to the company). Nevertheless, it can be quite easily explained: Microsoft wants to avoid antitrust issues from competing security giants.

An effective if somewhat slow utility, Microsoft Security Essentials is a solid contender in the free antivirus space. UPDATED WITH INDEPENDENT TESTS, October 1 2009.

Microsoft Security Essentials shows much promise. In our tests it was decent at detecting malware, particularly in proactive tests that simulate the handling of new, unknown malware. It took fourth place in our rankings of free antivirus software. The main drawback of the tool, due to launch on September 29, seemed to be its slow scan speed.

One thing we hope Microsoft will improve is Microsoft Security Essentials' relatively poky scan speed. It was the slowest in our on-access scan test, which judges how quickly scans run when you copy, open, or save files. The app's Dynamic Signature Service may account for some of that: When Security Essentials sees a potentially malicious file that doesn't match known malware, it contacts Microsoft servers for additional analysis. The feature likely affords greater protection owing to the use of the latest signatures online, but it may also introduce some delay if Security Essentials has to wait for a response.